| From: | "Nathan Boley" <npboley(at)gmail(dot)com> |
|---|---|
| To: | "Jeff Davis" <pgsql(at)j-davis(dot)com> |
| Cc: | "Zeugswetter Andreas OSB sIT" <Andreas(dot)Zeugswetter(at)s-itsolutions(dot)at>, "Gregory Stark" <stark(at)enterprisedb(dot)com>, "Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Proposal - improve eqsel estimates by including histogram bucket numdistinct statistics |
| Date: | 2008-06-10 18:46:02 |
| Message-ID: | 6fa3b6e20806101146m7c699be8qab6d74011435d051@mail.gmail.com |
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| Thread: | |
| Lists: | 503 와이즈 토토 페치 |
>> >> > One more problem with low ndistinct values is that the condition might very well
>> >> > hit no rows at all. But Idea 1 will largely overestimate the number of hits.
>>
>> Thats a good point, but I don't see a clear solution. Maybe we could
>
> I think that MCVs are the solution, right?
Only if they cover the entire range of values in the table.
> A low ndistinct means that those values will likely be MCVs.
Yes, but I don't think thats the point.
If we query on values that aren't in the table, the planner will
always overestimate the expected number of returned rows because it (
implicitly ) assumes that every query will return at least 1 record.
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